This disclosure relates generally to a capacitive pressure sensor, and more particularly to an improved sensor providing very precise and accurate measurements of pressure, particularly at very low (vacuum) pressures.
Pressure transducers have been employed in a myriad of applications. One such transducer is the capacitive manometer which provides very precise and accurate measurements of pressure of a gas, vapor or other fluid. Applications include precision control of vacuum based processes and semiconductor process control. Examples include semiconductor etch process and physical vapor deposition.
Capacitive manometers typically use (a) a flexible diaphragm forming or including an electrode structure and (b) a fixed electrode structure spaced from the diaphragm so as to establish capacitance there between. Variations in pressure on one side of the diaphragm relative to the pressure on the opposite side of the diaphragm causes the diaphragm to flex so that the capacitance between the electrode structure of the diaphragm and the fixed electrode structure varies as a function of this differential pressure. Usually, the gas or vapor on one side of the diaphragm is at the pressure being measured (Px), while the gas or vapor on the opposite side of the diaphragm is at a known reference pressure (Pr), the latter being at atmosphere or some fixed high or low (vacuum) pressure, so that the pressure on the measuring side of the diaphragm can be determined as a function of the capacitance measurement.
Many applications requiring extremely low pressures (high vacuum) have been and continue to be developed resulting in the need for capacitive manometers capable of measuring such low pressures. However, increasing the sensitivity of capacitive manometers to provide very precise and accurate pressure measurements at low pressures poses several design challenges. In order to measure extremely low pressures (high vacuum), capacitive manometers require very narrow gaps between the flexible diaphragm and the fixed electrode structure so that they can detect small changes in pressure.
A drawback to using very narrow gaps is that smaller changes in the shape of the electrode gap unrelated to the measurement of differential pressure across the diaphragm are also detected. One of these detrimental changes to the electrode gap shape is a change in the electrode gap spacing. Although it is common practice in the industry to reduce the effect of change in the electrode gap spacing by using the dual electrode design approach, good control over the electrode gap spacing provides further enhanced stability of the sensor output. This is especially important when one measures extremely low pressures (extremely small diaphragm deflections) enabled by the use of narrow electrode gaps.
Capacitance measurements are based on the well known equation for parallel plate capacitance C:C=ereoA/s, 
where C is the capacitance between two parallel plates,
eo is the permittivity of free space,
er is the relative permittivity of the material between the plates (for vacuum, er=1),
A is the common area between the plates, and
s is the spacing between the plates.
Based on this equation, one can derive the relationship that the fractional change in capacitance is equal to the negative of the fractional change in electrode gap spacing for each measuring electrode (ΔC/C=−ΔS/S).
It can then be readily seen that it is critical to maintain good control over the electrode gap spacing in order to provide stable control over the capacitance of each measuring electrode. In a simple dual electrode design, these effects are balanced to a first order at zero differential pressure for a flat diaphragm and electrode structure (each having different real values of flatness and inclination deviation from true plane) for a given electrical measurement technique such as with any number of commonly used bridge designs and other electrical measuring methods. Since, the sensor is configured to measure extremely low pressures (extremely small diaphragm deflections), just balancing the electrodes without making a stable electrode gap is not enough to reduce the uncertainty of the pressure measurement to adequately low levels in order to accomplish stable detection of the smallest pressures.
As the capacitive measurements are designed to detect changes in displacement between the fixed electrode structure and the diaphragm pressure resisting element, one source of error relates to any changes in the shape and position of the fixed electrode structure which produce changes in the sensor output that are unrelated to pressure.
In order to maintain good accuracy of the pressure measurement system, it is necessary to control critical aspects of the sensor geometry that may contribute to changes in sensor capacitance levels within the scope of the desired system accuracy.
Along the lines of repeatability and stability, for prior art capacitive pressure sensors of the clamped electrode support type, where no allowance has been made for the effects of differential expansion and lack of radial compliance between the electrode support and sensor housing, any slip occurring between these components under the action of temperature changes and sometimes ambient pressure changes is the result of large, built up shear forces that are subsequently released when they exceed the frictional binding forces in any of the clamped joints. The changes in these non-conservative forces result in mechanical hysteresis such that the relative lateral positions of the mating components generally do not repeat and cannot be predicted and compensated.
Some exemplary capacitance pressure sensor constructions are described in U.S. Pat. No. 6,105,436. These devices utilize either independent or integral “compliant rings” to relieve or reduce any mechanical strain from being developed between the electrode disk assembly and the sensor housing.
The aforementioned “integral” compliant ring does an effective job of providing good registration between the electrode disk and the diaphragm, and reducing mechanical strain in the electrode structures; but it is still limited by the lowest practical radial spring constant that can be attained by just simply reducing its thickness.
There is a need for a capacitive pressure sensor capable of improved electrode gap shape control at low pressure measurements so as to improve the measurement capabilities of the manometer at these lower pressures.
Reference is made to U.S. Pat. Nos. 7,757,563; 7,706,995; 7,624,643; 7,451,654; 7,389,697; 7,316,163; 7,284,439; 7,201,057; 7,155,803; 7,137,301; 7,000,479; 6,993,973; 6,909,975; 6,735,845; 6,672,171; 6,568,274; 6,105,436; 6,029,525; 5,965,821; 5,942,692; 5,932,332; 5,911,162; 5,808,206; 5,625,152; 5,271,277; 4,823,603; 4,785,669 and 4,499,773; and U.S. Patent Published Application Nos. 20090255342; 20070023140; 20060070447; 20060000289; 20050262946; 20040211262; 20040099061; all assigned to the present assignee.